Personally, I think it's overrated because it got 10/10, but became very boring about a month after first playing it.
I found it a chore to finish the story line, and once I did that I had very little desire to play it any more.
For me, once the initial 'wow factor' of the good graphics and detailed city had worn off, I found the game lacking the 'fun factor' which makes me want to keep playing a game.
I didn't really think the story was good as some people had claimed it to be. It took itself too seriously for a fun game, and in attempting to have it's cake and eat it, failed at being a deep, poignant story and at being a fun game.
For instance, one moment Niko is lamenting about the horrors of war he's seen in his past, and how he's trying to change and lead a better life, the next moment he's agreeing to murder total strangers because another total stranger on the street wants him to buy some crack.
One of the themes of the game seemed to be how Niko and Roman were trying to make it big in America, make a better life for themselves. Before I even finished the game I had over 5 million dollars, so what were they yammering on about? They already were rich beyond their wildest dreams, except Niko was stingy and only wanted to spend his millions on clothes, hot-dogs and weapons.
The story would have been believable and had worked if the video game elements were removed from the game, likewise the game side of it would have worked better if the story wasn't dragging it down. Rockstar said that the left a lot of the fun features from previous games out of GTA IV because it would be unrealistic and out of character, Niko wouldn't do those things.
It also seemed unfinished, like one of the tutorials teaches you how to perform a 1 punch KO, and that's the only time in the game you can perform that move, what was the point? The same with the camera phone, apart from the few missions where you needed to take photos, what was the point? You could take photos any time you wanted, but you couldn't save them, unlike San Andreas.
I think ultimately though, my expectations for GTA IV had been spoiled by other games
Scarface: The World Is Yours spoilt my expectations about what a good, cinematic sandbox game should be like, and Bully and Saint's Row spoilt my expectations of what a good, fun sandbox game should be like.
I think if I hadn't played any other sandbox game since GTA III, and hadn't read any hype and press releases from Rockstar, prior to GTA IV, I would have been more impressed.
It's not all bad though, I did like the driving physics and found the cop missions really fun. Plus the darts game was pretty good, and I loved going drinking, it's just a shame you couldn't get drunk outside the man-dates.
Anyway, that's just my opinion of the game, I'm sure lots of people love that game and think it is worth 10/10, and they wouldn't be wrong in their opinions, after all I can't dictate to them what they find fun.